Today, there are two commercial processes in widespread use for the production of long chain alkylaromatic hydrocarbons. Both of these processes involve alkylating an aromatic compound, generally benzene, with a linear olefin in the presence of a homogeneous acid catalyst in a stirred batch reactor. In one process, the catalyst is AlCl3 which is washed out of the reactor with waste water at the end of the reaction. In the other process the catalyst is HF which is distilled off and recovered for re-use at the end of the reaction. However, both of these processes suffer from a number of serious disadvantages. Firstly, both of the catalysts are very corrosive and have numerous safety and environmental issues associated with their use. Secondly, the surfactant industry has an increasing demand for alkylaromatic hydrocarbons in which the aromatic species is predominantly located at the 2- and 3-positions in the alkyl side chain, since 2- and 3-phenyl substituted alkylbenzene sulfonates exhibit superior performance as surfactants. However, alkylaromatic hydrocarbons made with AlCl3 as the catalyst typically contain about 52% of the 2- and 3-phenyl isomers, whereas materials produced using HF as the catalyst generally only contain about 17% of the 2- and 3-phenyl isomers.
Molecular sieves, such as aluminosilicate zeolites, are also known to be effective acid catalysts for the production of alkylaromatic hydrocarbons, particularly short chain alkylaromatic hydrocarbons, such as ethylbenzene and cumene. In addition, molecular sieve catalysts are well known to offer significant advantages over AlCl3 and HF, in that they are generally non-toxic and non-corrosive. Moreover, when used to alkylate aromatic compounds with linear and lightly branched long chain olefins, certain molecular sieves have been found to be effective in producing alkylaromatic hydrocarbons with very high levels, up to 95%, of the 2- and 3-phenyl isomers. See, for example, U.S. Pat. No. 5,026,933 and U.S. Patent Application Publication No. 2005/0165264.
However, molecular sieve catalysts suffer from a number of disadvantages which has to date limited their use as alkylation catalysts for the production of long chain alkylaromatic hydrocarbons. Firstly, to impart structural stability, molecular sieve catalysts are normally used as extrudates, typically in combination with a binder. As a result, molecular sieve catalysts tend to have limited surface area and hence activity, so that the alkylation reaction (normally conducted in a fixed bed tubular reactor) typically takes on the order of 24 hours to achieve at least 90% conversion of the olefin feed. Moreover, molecular sieves are generally hydrophilic and so must be dehydrated by calcination before use and, after dehydration, must be protected from the air to prevent absorption of water and resulting reduced activity.
Secondly, current molecular sieve catalysts are unsuitable for use in the stirred batch reactors used for the commercial production of long chain alkylaromatic hydrocarbons using homogeneous catalysts. In particular, the surface area of extrudates of molecular sieve catalysts is too small to produce any appreciable rate of reaction in a stirred reactor process. Further, the extrudates would tend to separate from the liquid phase reaction mixture and fall to the bottom of the reactor. Crushing the extrudates to reduce their particles size could be used to increase their surface area and activity. However, the resultant powder could not be calcined in a stirred batch reactor since, removing the water would generate a build-up of static electricity causing the catalyst to adhere to the walls of the reactor rather that being dispersed in the reaction mixture.
According to the present invention, there is provided a method of producing and using molecular sieve-containing catalyst particles suitable for use in the production of alkylaromatic hydrocarbons, particularly long chain alkylaromatic hydrocarbons, in, for example, stirred tank reactors. In this method, a mixture comprising molecular sieve crystals are formed into catalyst particles having an average cross-sectional dimension between about 0.01 mm and about 3.0 mm. The catalyst particles are coated with a liquid paraffin inert to the conditions employed in the alkylation reaction. The catalyst particles may also be combined with a binder before being formed and then calcined to remove water. The small size of the catalyst particles ensures sufficient activity for use, for example, in a stirred batch reactor, while the paraffinic coating helps to protect the particles against water adsorption prior to use as well as assisting in dispersion of the particles in the alkylation reaction medium.
It is to be appreciated that there are numerous references in the patent and academic literature to stirred reactors being used in bench-scale testing of zeolite catalysts in the production of long chain alkylaromatic hydrocarbons, see, for example, Example 3 of U.S. Pat. No. 6,583,096. However, in view of the problems discussed above, zeolite catalysts particles having an average cross-sectional dimension between about 0.01 mm and about 3.0 min have to date been considered unsuitable for use in commercial-scale, in, for example, stirred tank reactors, for producing long chain alkylaromatic hydrocarbons.